This study evaluated the effects of concurrent strength and endurance training in active male recreational endurance runners (N = 28). Ss were divided into three strength training groups: maximal (N = 11), explosive (N = 10), and circuit training (N = 7). Ss completed six weeks of common preparatory strength training and then eight weeks of specific strength training followed by 14 weeks of intentionally reduced strength training and increased endurance training. Strength training (including at least two exercises for leg extensors) occurred on average ~1.3, ~1.5, and ~0.5 times per week in each training period, respectively for each group. Endurance training volume (running km) increased progressively throughout the study. Concentric leg- press strength (1 RM), jumping power (countermovement jump), muscle activation (EMG of VL + VM), and endurance performance (vVO2, VO2max, and running economy) were measured before the preparatory strength training, at 0, 4, and 8 weeks of training, and at the end of the reduced strength training period.

During the preparatory and strength training, 1 RM leg-press improved significantly in all groups peaking in the fourth week, while significant gains in jumping power peaked at week 8. Gains in 1 RM were accompanied by increased EMG activity in the maximal and explosive groups but not in the circuit training group. VO2max increased only in the maximal strength group by the eighth week of strength training. vVO2 improved significantly in all groups by the eighth week and running economy improved in the explosive group. During and following reduced strength training, progressive decreases were observed in 1 RM, jumping power, and EMG activity. Significant increases in vVO2 in the maximal and explosive groups and running economy in the maximal group, continued after the strength intervention ceased.

Implication. Maximal and explosive strength training performed concurrently with high volume endurance training led to increases in strength, jumping power, and EMG activity but strength development plateaued between four and eight weeks. At the same time, the continued endurance training led to progressive gains in VO2max, vVO2, and running economy. A reduction in strength training resulted in decreases in the strength gains. While strength was decreasing, vVO2 and running economy continued to improve in the maximal and explosive strength groups. Adaptations to strength training were maintained to some extent and did not fall below starting values. Despite the authors' attribution of endurance gains being stimulated by the strength work, a clearer interpretation of the results of this study is that endurance and strength responses can occur concurrently and independently.